This invention concerns a process for recovering maleic anhydride from gaseous compositions which comprise water as well as maleic anhydride. The process is carried out by condensing the gas under select temperature and pressure conditions. In this way, maleic anhydride is collected as a condensate while light impurities such as water and acids remain in the vapor phase.
This process is particularly adaptable to the recovery of maleic anhydride which is normally lost during the light stripping of crude maleic anhydride. Crude maleic anhydride is produced by the vapor phase oxidation of a hydrocarbon feed, and is recovered by absorption from the oxidation effluent using a liquid solvent followed by distillation of the anhydride-rich absorbent. U.S. Pat. No. 3,818,680 to Marquis thoroughly describes a typical process for producing crude maleic anhydride using a liquid intramolecular carboxylic acid absorbent. Other organic absorbents can be used such as those disclosed in U.S. Pat. Nos. 2,574,644; 3,040,059; 2,893,924; 3,891,680; and 3,850,758.
It is conventional practice to further refine the crude maleic anhydride by stripping off low-boiling impurities using reduced pressure distillation. In general, the low-boiling impurities comprise carboxylic acids such as acetic and acrylic acids and water. However, under even the best of operating conditions, the overhead vapor will also contain some maleic anhydride which is either lost through the vacuum system or recovered through an elaborate absorption and recycle process. Thus, the overhead vapor comprises maleic anhydride and water. Conventional absorption processing has been tried as a method of recovering the anhydride, but has several drawbacks. In particular, a significant proportion of the impurities must also be absorbed and recycled to insure complete anhydride recovery. Thus, using absorption to recover the anhydride from the light stripper overhead vapor is a relatively inefficient process.
Accordingly, it would be advantageous to provide a process for recovering maleic anhydride from gaseous mixtures also containing water and low-boiling impurities which while recovering the anhydride leaves the water and other impurities in the gas.
Several U.S. patents discuss the recovery of maleic anhydride from gaseous compositions also containing water. In general, these methods use partial condensation of maleic anhydride effected by cooling the gaseous composition to temperatures above 50 .degree. C. so as to minimize condensation of water. Such practice, however, results in only partial recovery of maleic anhydride contaminated with some maleic acid, since any attempt at complete recovery requires cooling the gas below the water dew point. If the water dew point is reached, the maleic anhydride will hydrolyze to maleic and fumaric acids. Thus, for example, British Pat. No. 822,612 teaches diluting the effluent gas from the oxidation reactor with an inert gas until the anhydride partial pressure is less than 2.24 millimeters of mercury and cooling the gas to about 20.degree. C. Similarly, U.S. Pat. No. 2,762,449; U.S. Pat. No. 2,812,037; and French Pat. No. 1,303,126 teach partial condensation processes for recovering maleic anhydride by cooling the gas to temperatures between 50.degree.-60.degree. C. from gas compositions comprising the anhydride and relatively low concentrations of water. In summary, prior art teaches recovery of maleic anhydride by cooling maleic anhydride gases at about one atmosphere pressure.
The prior art partial condensations are particularly useful where the concentration of anhydride is relatively high and concentration of water relatively low, usually less than 5 volume %, such that the over-all efficiency of the recovery is not significantly reduced by leaving minor amounts of anhydride in the gas. However, it is still desirable to provide a process which provides essentially complete recovery of anhydride even where the concentration of water is relatively high, typically more than 10 volume %.